linux-kernel - Re: NO_HZ_FULL and tick running within a reasonable amount of time

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Message-ID: <20180403092328.GA4082@hirez.programming.kicks-ass.net>
Date:   Tue, 3 Apr 2018 11:23:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, frederic@...nel.org,
        cmetcalf@...lanox.com, cl@...ux.com, lcapitulino@...hat.com,
        efault@....de, riel@...hat.com, tglx@...utronix.de,
        kernellwp@...il.com, mingo@...nel.org
Subject: Re: NO_HZ_FULL and tick running within a reasonable amount of time

On Mon, Apr 02, 2018 at 03:04:38PM -0700, Paul E. McKenney wrote:

> The WARN_ON_ONCE() triggering is this guy:
> 
> 	delta = rq_clock_task(rq) - curr->se.exec_start;
> 	WARN_ON_ONCE(delta > (u64)NSEC_PER_SEC * 3);
> 
> But given that ->se.exec_start is zeroed from time to time, for example,
> in migrate_task_rq_fair(), I am a bit suspicious of this check.
> 
> What am I missing here?

We clear it on migration, but set it when we schedule a task back in.
The above checks that the 'current' task of that CPU had a tick at least
3 seconds ago (to ensure tasks don't go too long without ticks).

The 'current' task is obviously scheduled in and thus must have !0
exec_start time.